High‐performance thermoelectric materials require simultaneous reduction of thermal conductivity and electrical resistivity, among other criteria. Here it is shown that the introduction of Na2CO3 into the melt‐route fabrication process for the well‐known thermoelectric Cu2Se has a beneficial and surprisingly strong effect. There is a significant enhancement in electrical conductivity which density functional theory calculations suggest may be due to the effect of Na and O doping in the Cu2Se matrix. There is also a 34% reduction in thermal conductivity which is likely due to a high density of defects causing scattering of phonons. Overall, however, there is only relatively a small change in Seebeck coefficient. A higher power factor of 12.6 µW cm−1 K−2 is achieved versus 8.8 µW cm−1 K−2 for standard Cu2Se. A very high value of zT of 2.3 is obtained at 804 K versus 1.1 for standard Cu2Se.